The present invention relates to drilling subterranean well bores of the type commonly used for oil or gas wells. More particularly, this invention relates to an improved method and system for regaining hydrostatic fluid pressure control of a well bore after the well bore receives an influx of fluid from the formation. The methods and system of this invention may facilitate more timely circulation of the fluid influx out of the wellbore while circulating a more dense fluid into the well bore to regain hydrostatic control of the well bore.
Drilling subterranean wells typically includes circulating a drilling fluid (xe2x80x9cmudxe2x80x9d) through a drilling fluid circulation system (xe2x80x9csystemxe2x80x9d). The circulation system may include a drilling rig and mud treating equipment located substantially at the surface. The drilling fluid may be pumped by a mud pump through the interior passage of a drill string, through a drill bit and back to the surface of the well bore through the annulus between the well bore and the drill pipe.
A primary function of drilling mud is to maintain hydrostatic fluid pressure control of fluids in the formations penetrated by the well bore. Weighting agents may be added to a mud to achieve the desired mud density. Traditional overbalanced drilling techniques typically practice maintaining a hydrostatic fluid pressure on the formation equal to or slightly overbalanced with respect to formation fluid pressure (xe2x80x9cpore pressurexe2x80x9d), both when circulating and when not circulating the mud. In underbalanced drilling techniques, hydrostatic pressure in the well bore is maintained at least slightly lower than formation pore pressure by the mud, supplemented with surface well control equipment. If the wellbore encounters a zone having a higher pore pressure than the hydrostatic fluid pressure in the mud, an influx of formation fluid may be introduced into the wellbore. Such occurrence is known as taking a xe2x80x9ckick.xe2x80x9d
In the well bore drilling industry, it is common practice to frequently during the course of drilling the wellbore, measure and record slow mud pump rates and corresponding pump circulation pressures required to circulate the mud at the reduced rate with the mud pumps. Such measurements may be made at such rates as may be used in circulating a kick out of the well bore, e.g., one-half to one-third of the normal circulation rate. Additional determinations may also be performed, including the cumulative number of pump strokes required to circulate the hole.
When a kick is taken, the invading formation liquid and/or gas may xe2x80x9ccutxe2x80x9d the density of the drilling fluid in the well bore annulus, such that as more formation fluid enters the wellbore, hydrostatic control of the wellbore may be lost. Such occurrence may be noted at the drilling rig in the form of a change in pressure in the wellbore annulus, changes in mud density, and/or a gain in drilling fluid volume in the mud system tanks (xe2x80x9cpit volumexe2x80x9d).
Typically when a kick is detected or suspected, mud circulation is halted and the well bore closed in/shut in to measure the pressure buildup in the well bore annulus, pit gain and shut-in drill pipe pressure. Appropriate well-killing calculations may also be performed while the well is closed in. Thereafter, a known well killing procedure may be followed to circulate the kick out of the well bore, circulate an appropriately weighed mud (xe2x80x9ckill mudxe2x80x9d) into the well bore, and ensure that well control has been safely regained.
One of the most common techniques for killing the well and circulating an appropriate kill fluid is the xe2x80x9cconstant bottom hole pressurexe2x80x9d method, whereby bottom hole pressure may be maintained substantially at or above formation pore pressure. Two variations of this method exist. The first variation may be known commonly as the xe2x80x9cDriller""s method.xe2x80x9d The Driller""s method may be utilized when kill weight fluid is not yet available for circulation. In the Driller""s method, the original mud weight may be used to circulate the contaminating fluids from the well bore. Thereafter, kill weight mud (xe2x80x9cKWMxe2x80x9d) may be circulated into the drill pipe and the well bore. Although two circulations may be required to effectuate the driller""s method, the driller""s method variation may be quicker than the subsequently discussed variation.
The second variation of the Constant Drill Pipe Pressure method may be commonly known as the xe2x80x9cwait and weightxe2x80x9d method, or the xe2x80x9cEngineer""sxe2x80x9d method. In the xe2x80x9cwait and weightxe2x80x9d method, KWM is prepared and then circulated down the drill string and into the well bore to remove the contaminating fluids from the well bore and to kill the well, in one circulation. This method may be preferable in that this method may maintain the lowest casing pressure during circulating the kick from the well bore and may thereby minimize the risk of damaging the casing or fracturing the formation and creating an underground blowout.
A substantially constant bottom hole pressure may be maintained in both methods. In either method, pressure on the casing and/or drill pipe may be controlled by adjusting a choke conducting mud from the casing to a mud reservoir. In addition, to further control pressure the mud pump rate may be maintained at one of the previously measured rates and corresponding pressures. In the Driller""s method, a constant drill pipe pressure may be maintained during the first circulation, which may include the shut-in drill pipe pressure (xe2x80x9cSIDPPxe2x80x9d) plus the slow rate pump pressure, plus a nominal safety factor, e.g., fifty psig. During the second circulation, the casing pressure may be held constant while the KWM is circulated to the bit, and then the drill pipe pressure held constant while the KWM is circulated from the bit to the surface.
In the xe2x80x9cwait and weightxe2x80x9d method, a substantially constant bottom hole pressure may be maintained during the one circulation of KWM. KWM may be circulated down the drill string while maintaining drill pipe pressure at a calculated, pre-determined pressure schedule while the mud pump is maintained at a constant rate. The drill pipe pressure may gradually decrease as KWM is circulated to the bit. After KWM reaches the bit, the drill pipe pressure may be held constant until the KWM reaches the surface.
A combination method is known which may combine portions of each of the above two methods. After the well is shut-in and the pressures recorded, pumping of original weight mud may begin while the original weight mud is being weighted up to KWM, as the kick is being pumped out of the well bore.
Each of the aforementioned methods may be time consuming and may require extensive planning, calculations, monitoring, human intervention and/or coordinated regulation of components, rates and pressures during execution of the respective method. In addition, each method typically uses a substantially constant pump rate in order to maintain control of the process during execution of the respective method. The wait and weight method also may require constructing a graphical or tabular pumping schedule of pump pressure versus volume pumped, to follow during the procedure. Further, in the event it becomes necessary to change pumping rates and/or interrupt pumping during execution of the kill procedure, it frequently may be necessary to record new shut-in pressures, new circulating pressures, and recalculate a new pumping and/or pressure schedule. A key component of each method may be adhering to a substantially constant pump rate during the procedure and maintaining a substantially constant bottom hole pressure.
Typically, the intent of the operator is to hold pump rate constant, and only change the pump rate after circulation has started if some excessive or undesirable condition arises. For example, when a circulated kick enters long, narrow, and/or restrictive choke lines, such as may be encountered with a deepwater floating rig. In anticipation of this, the operator may collect slow circulation data at up to three discrete rates.
Following completion of the kill procedure, new pressure readings should be taken, wherein the well may be under hydrostatic control, such that the casing pressure may read substantially zero psig. In the event the shut-in casing pressure and/or drill pipe pressure may not be zero psig, it may be necessary to repeat the kill procedure. A kill procedure may be deficient at completely regaining well control due to inaccurate previous pressure readings, changes in pumping rate during execution resulting in an influx of additional contaminating fluids, and/or otherwise failing to maintain a substantially constant bottom hole pressure in excess of formation pore pressure. A failure to maintain a constant bottom hole pressure may result from miscommunication, erroneous operation of the choke, procedural miscalculations, and/or other inappropriate equipment operation during the procedure.
The amount of human intervention required, including the substantial gathering of rate and pressure information, calculating and scheduling a kill procedure, maintaining a constant pump rate, and coordinating the operation of equipment to maintain the appropriate surface pressures and constant bottom hole pressure are each disadvantages of the prior art.
An improved method is desired for conducting a well killing procedure in a more timely fashion and with greater precision and efficiency than may be possible under existing methods. A method is also desired which may provide for varying the pump rate during the kill procedure without having to shut down and determine a revised pressure and rate pumping schedule.
The disadvantages of prior art are overcome by the present invention. An improved method and system for more accurately controlling well bore hydrostatic pressure are described herein.
This invention has particular utility in controlling hydrostatic and formation pressures within a well bore. More particularly, this invention may facilitate improvements over prior art in facilitating regaining hydrostatic control of the well bore in a more timely fashion and with improved process control therein. This invention provides methods and systems for circulating a kick out of a well bore and regaining hydrostatic control of the well bore with the ability to vary the pump rate. Thereby, a kick may be circulated from within the well bore and KWM circulated, both in a more timely fashion and with improved process control, as compared to prior art.
A control system may be provided which may monitor and/or record one or more selected drilling parameters and which may also provide automated control of a kill procedure. During drilling the well bore, on a regularly scheduled basis, such as each day, or each crew change, or each particular footage drilled, the control system may obtain and record information pertaining to selected drilling parameters which may be useful in executing a well killing procedure. The control system may record selected pressures, pump rates, and pit volumes in the mud system. Thereby, when a kick is taken, the control system may be relied upon to effectively determine the procedure for circulating the kick from the hole and for circulating the KWM, and then to controllably execute the procedure.
In addition, the control system may facilitate selectively modifying the kill procedure in response to changes or interruptions in the pumping schedule. Thereby, the pump rate utilized during execution of the kill procedure may be selectively varied and/or interrupted while maintaining a substantially constant bottom hole pressure, at or above the formation pore pressure.
It is an object of this invention to provide methods and systems for regaining hydrostatic control of a well bore subsequent to taking a kick, by pumping at a variable pump rate.
It is a feature of this invention to provide methods and systems for regaining hydrostatic control of a well bore subsequent to taking a kick by closing in the well bore annulus with a BOP and a choke on a return fluid flow line, and use a programmable controller to operate at least the choke to maintain a substantially constant bottom hole pressure within the well bore while pumping a kill fluid.
It is a feature of this invention to routinely at selected intervals, automatically measure and record drill pipe circulation pressure for a range of mud pump circulation rates.
It is also a feature of this invention to thereby determine the appropriate drill pipe circulation pressure required to maintain a substantially constant bottom hole pressure, at any point in the kill procedure and at any circulation rate which may be in effect at the time.
It is a feature of this invention to selectively use any of a wide continuum of circulation rates while circulating a kick out of the wellbore, and to vary the rate as desired while pumping the kick out. Selection of pump rate may be made manually by an operator, or automatically by a control system, or both.
It is an advantage of this invention to utilize a control system and sensed measurements of one or more drilling parameters to monitor, control and execute the kill procedure.
It is also an advantage of this invention to expedite circulating a kick out of a well bore, thereby decreasing the time required to regain well control and decreasing well bore drilling costs.
It is further an advantage of this invention to improve control of operable equipment during the procedure by utilizing a control system to regulate pump rates and choke position.
Yet another advantage of this invention is to decrease the potential for creating too much hydrostatic pressure in the well bore and fracturing the formation.
It is an additional advantage of this invention to improve the safety of circulating a kick from a well bore and killing the well, by utilizing a programmable control system. The control system may consider sensed measurements of well bore and drill string pressures, circulation rates, mud weight, well bore dimensions, and thereby determine an optimum kill procedure and thereafter controllably execute the procedure with reduced potential for miscalculation or manual control errors.
These and further objects, features, and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to figure in the accompanying drawing.